htlcswitch/link_test: increase fwdpkg gc interval

In this commit, we increase the fwdpkg gc interval
to avoid having it conflict with switch tests that
inspect forwarding packages. The current timeout is
a little too short on travis, and sporadically fails
TestChannelLinkCleanupSpuriousResponses, which was
added recently.

htlcswitch/link_test.go

@@ -1560,7 +1560,7 @@ func newSingleLinkTestHarness(chanAmt, chanReserve btcutil.Amount) (
 		Registry:       invoiceRegistry,
 		ChainEvents:    &contractcourt.ChainEventSubscription{},
 		BatchTicker:    bticker,
-		FwdPkgGCTicker: ticker.MockNew(5 * time.Second),
+		FwdPkgGCTicker: ticker.MockNew(15 * time.Second),
 		// Make the BatchSize and Min/MaxFeeUpdateTimeout large enough
 		// to not trigger commit updates automatically during tests.
 		BatchSize:           10000,